1. Field of the Invention
This invention relates generally to filters for use in subterranean environments, such as in oil and gas wells, water wells, and geothermal wells, and for other applications in which it is desired to remove a liquid or gas from the ground without bringing soil particles, such as sand or clay, up with the liquid or gas. In particular but not exclusively, it relates to filters which can be formed from pipes having externally upset ends.
2. Description of the Related Art
Filters are frequently used in subterranean wells in order to remove undesirable particles from liquids or gases produced by the wells. Typical particles which need to be filtered out are sand and clay, which can be highly damaging if allowed to enter into pumps and other well production equipment along with the production fluid.
A typical well filter includes a filter body, containing a filter medium, mounted on a length of well pipe which serves as an inner support member for the filter body. The lengthwise ends of the filter body are protected and sealed to the pipe by annular end connectors. The lengthwise ends of the well pipe are equipped with threaded portions by means of which the filter can be connected to a string of pipe, to a down-hole pump, or to other well equipment.
When a pipe string within a well is subjected to high stresses, a failure phenomenon referred to as "jump out" may occur in which the mating threads of adjoining lengths of the pipe making up the string become disengaged, permitting well fluid to flow into the pipe string where the jump out took place. This phenomenon is particularly common in horizontal or highly deviated wells, in which the pipe string is subjected to high bending loads at the connections between pipes. The likelihood of jump out can be reduced by the use of pipe having upset ends. Upset ends refers to end portions of a pipe having a greater wall thickness than the body of the pipe. A pipe having one or more upset ends is sometimes referred to as an upset pipe. The increased wall thickness of an upset end produces a difference in the inner or outer diameter of the upset end relative to that of the body of the pipe. If the maximum outer diameter of the upset end is greater than that of the body of the pipe, the upset end is described as being externally upset. If the minimum inner diameter of the upset end is less than that of the body of the pipe, the upset end is described as being internally upset. An upset pipe may be both externally and internally upset at the same time, in which case the maximum outer diameter is greater than that of the body of the pipe while the minimum inner diameter is less than that of the body of the pipe. In the present description, "externally upset" will be used to refer to upset ends which have an increased outer diameter relative to the body of the pipe, whether or not the ends are also internally upset.
Various modes of failure besides jump out can also be prevented or made less likely by the use of pipe with upset ends. In addition, lengths of pipe with upset ends can be connected directly to each other without the use of a coupling, so the sealing integrity of the connections between lengths of pipe with upset ends can be higher than the sealing integrity of connections employing couplings because of fewer locations in which a seal needs to be formed. For these reasons, it is highly advantageous to use a pipe having upset ends as an inner support member for a well filter.
However, conventional well filters are designed to be assembled by sliding the filter body, end connectors, and other components of the filter over one of the ends of the inner support member of the filter. Therefore, no portion of the inner support member can have an outer diameter which is larger than the minimum inner diameter of the filter body and other members to be installed on the inner support member. This precludes the use of pipe with externally upset ends as an inner support member for a conventional well filter.